求助，怎么使用关联 [ 新手入门 ]

public function getProjectNumberList($pid){
$ProjectNumbers=Machine::model()->findAllByAttributes(array('machine_sn'=>$pid));
 foreach($ProjectNumbers as $ProjectNumber){
$listData[$ProjectNumber->project_number]=$ProjectNumber->projectRelation->project_number;
}
return CHtml::listData($ProjectNumbers, 'machine_sn', 'project_number');
}

在模型里面用了relation,然后这样写的时候就报错：Trying to get property of non-object